Fabric-reinforced elastomeric tanks are often used for storing fuel or other liquids. These tanks may be required to hold fuel for extended periods of time while exposed to harsh environmental conditions, but then may be folded and stored for years before being used again.
The tanks may deteriorate as a result of being stored in warehouses having extreme climates. Heat and humidity are harmful to the elastomers commonly used for the tanks. As polymers undergo environmental aging, the surface chemistry is the initial area of breakdown and change, which includes oxidative and hydrolytic attack. This alters the surface free energy. The surface resistivity drops incrementally and in proportion to the degree of oxidative and hydrolytic attack. Also, although the tanks are drained after use, residual fuel usually remains in them, which can degrade the polymer over time. For handling ease and to save warehouse space, the tanks may be tightly folded and packed into crates. The stress concentrations caused by the folds can lead to premature failure of the tanks.
Accordingly, a need has arisen for a method of determining the condition of polymeric fuel tanks so that unreliable tanks can be taken out of service. A change in surface resistivity of the polymeric portion of a tank could mean a tank is unfit for service. However, the surface resistance of a polymer material is typically very high, and conventional low-voltage ohmmeters are not sufficiently sensitive. Conventional high-voltage ohmmeters usually apply a potential on the order of several hundred volts across the specimen. However, when measuring the surface resistance of fuel tanks, there is a high probability that flammable fuel vapors will be present in the area. In this environment, an electrical potential of large magnitude could produce a spark of sufficient energy to cause fuel-vapor ignition. Furthermore, there is an electrical shock hazard to personnel.